Be careful, the actual logging can happen at a later time, and because the log function may take a reference to the value, if you modify r it may show the modified version of r in the logging instead of the original r.

Using -F with tail is even better than -f because it handles files getting truncated or getting created.

This looks so over-engineered. Most of the time you only need an error message. Make the message clear enough so that it can be shown to the end user.